The invention relates to controlling refrigeration.
A refrigeration system is used to cool an enclosed area, such as a food storage case, by absorbing heat from the enclosed area and releasing the heat outside the enclosed area. To accomplish this, a highly-pressurized liquid refrigerant (e.g., freon) is introduced into a low pressure environment provided by the interior of an evaporator coil (located within the enclosed area). Inside the evaporator coil, the refrigerant depressurizes, absorbs heat from the enclosed area, and boils, forming a vapor. This vapor is removed from the evaporator coil and compressed back into the liquid refrigerant inside a condenser coil (located outside of the enclosed area) where the refrigerant releases the absorbed heat.
An expansion valve typically controls the flow of liquid refrigerant into the evaporator coil. As a result of a high to low pressure transition across the evaporator coil and the increased volume presented by the evaporator coil to the refrigerant entering the coil, the liquid refrigerant changes (inside the coil) to a saturated vapor (a mixture of liquid and gas at the refrigerant boiling point). Since a liquid at its boiling point does not increase in temperature until a change to the vapor state has taken place, it is desirable to maintain a sufficient flow through the expansion valve such that the refrigerant remains a saturated vapor throughout the evaporator coil. However, to avoid damaging most compressors, the refrigerant needs to be completely in a vapor state when returning to the compressor system. Thus, a typical control unit attempts to operate the valve in a manner to ensure this state change by creating a refrigerant temperature rise (often referred to as a superheat of the coil) across the evaporator coil.
The control unit typically measures the superheat and regulates the valve based upon this measurement. For purposes of preventing undershoots in the superheat (and the related problem of returning liquid refrigerant to the compressor), a typical solution is to target a higher than desirable superheat.